1. Field of the Invention.
This invention is directed to top drives for well operations including well drilling and particularly self-aligning top drives.
2. Description of Prior Art.
In many instances the drilling of a well must be conducted in a body of water. A drilling rig known as a "jack-up" is often used. A jack-up drilling rig utilizes three or four legs which are powered down to the bottom of the body of water. Upon reaching bottom the legs are further extended, thereby elevating the derrick and the supporting structure out of contact with the water which transfers all of the weight to the support legs.
Upon initial drill site installation the drilling floor is jacked essentially level through individual leg jacking. However after a passage of time, because of inconsistency of the bottom surface, the drilling structure may often assume a non-level position due to the settling of one or more legs.
Since the drill string is suspended from the top of the derrick, the axis of the pipe will tend to remain at a true vertical with respect to the bottom surface, but the axis of the derrick will be displaced from the vertical an amount in proportion to the uneveness of the drill floor.
It is common practice to mount two elongated guide rail members rigidly to the derrick means and on the true center line of the well axis. Mounted within the elongated members (guide rails) is usually found a top drilling drive and guide dolly.
Since the top drive dolly will tend to follow the guide rail whenever the drilling rig departs from a level posture, a considerable side load is created on the dolly and also the top drive central threaded shaft when hoisting a drill string of a considerable weight. This happens because the drill string will tend to remain vertical. Since the top drive must support this loading, considerable unnatural reaction forces must be absorbed and abnormal stresses are placed upon the top drive shaft, the guide rails and the guide dolly.
Prior art top drives do not effectively address this problem and have experienced many failures in the field, such as guide rail failure, dolly guide roller failure, and most serious, and often catastrophic, failure of the drill string connection brought about by side loading which causes metal fatigue of the tool joint threaded connections.
In accordance with .sctn.1.56 of 37 C.F.R. the following are disclosed: